Multipactor switch



Nov. 2l, 1967 K. L. HORN 3,354,349

MULTIPACTOR SWITCH v Filed Dec. v, 1964 2 Sheets-Sheet l United States Patent O 3,354,349 MULTIPACTOR SWITCH Kenneth L. Horn, Hawthorne, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Fiied Dec. 7, 1964, Ser. No. 416,445 3 Claims. (Cl. 315-39) ABSTRACT F THE DISCLGSURE The disclosed microwave power switch includes a waveguide having a pair of opposing secondary electron emissive surfaces between which multipactor action may be established in response to input microwave power above a preselected level. An external electron gun supplies suiiicient electrons to the waveguide to insure immediate commencement of multipactor action, and a wire screen mounted across a waveguide aperture adjacent the electron gun improves the impedance match between the waveguide and the gun, in addition to shielding the waveguide from gun generated noise.

This invention relates to microwave power switching, and more particularly relates to a multipactor switching device for microwave power in which a separate electron source is provided to insure immediate initiation of multipactor action.

In certain types of radar systems a T-R (transmitreceive) switch is employed to block microwave signals above a given power level, while passing signals below the given power level, thereby preventing excessive bursts of power from destroying the receiving equipment. One type of device which has been `employed for this purpose is a gas discharge switching tube. In the operation of such tubes a substantial recovery time during which signals cannot be received exists after each discharge; hence the pulse repetition rate of systems incorporating this type of switch is severely limited.

Another type of device which has been employed for microwave power switching utilizes a secondary electron resonance phenomenon termed multipacton In multipactor devices a radio frequency electric field is applied to an evacuated chamber including a pair of spaced opposing surfaces each having a secondary electron emission coeicient greater than unity. If the radio frequency electric field is of suliicient amplitude and if the frequency of the electric field is properly synchronized with the surface spacing, electrons will be emitted from one surface and accelerated toward the opposite surface where they will arrive when the electric field reverses its polarity. Secondary electrons will be emitted from the opposite surface, and on account of the greater than unity secondary emission coefiicient more electrons will be emitted from the opposite surface than impinged upon it. Since the electric field reverses its polarity as the secondary electrons are emitted, these secondary electrons will be accelerated back to the first surface from which they will release secondary electrons coincident in time with another polarity reversal of the electric field. Thus, the process continues as electrons are accelerated back and forth between the surfaces in time synchronization with the alternating electric field. The net result is to establish multipactor action, i.e., electron multiplication between a pair of secondary electron emissive surfaces by means of a time synchronous alternating electric field.

The aforedescribed phenomenon may be utilized to provide radio frequency power switching because when the input power to a multipactor switch is greater than the level required to sustain multipactor action, radio frequency power is absorbed by the electrons and is dissipated when these electrons strike the secondary emissive surfaces, thereby limiting the power transmitted through the switch to a given level.

Although microwave switches of the multipactor type provide a recovery time several orders of magnitude shorter than that of gas discharge switching tubes, multipactor action does not necessarily commence immediately after the application of radio frequency power to the switch. Thus, during the turn-on of prior art multipactor switches a time delay exists during which excessive power can be passed to the receiver.

Accordingly, it is an object of the present invention to provide a multipactor switching device for microwave power in which multipactor action commences immediately after microwave power above a given level is applied to the device.

IIt is a further object of the present invention to provide a power switching device for electromagnetic waves which not only possesses an essentially instantaneous turn-on, but which also exhibits a minimum recovery time.

It is a still further object of the present invention to provide an improved multipactor T-R switch for use in a radar system which not only insures that excessive power transients will not damage the system, but which switch is also simple, reliable and durable.

In accordance with the foregoing objects, the presentinvention provides a switching device fortranslating electromagnetic waves of less than a preselected power level with substantially no attenuation and for reducing the power level of electromagnetic waves of more than the preselected power level to substantially the preselected level. A waveguiding structure is provided including at least a pair of spaced apart secondary electron emissive surfaces at least portions of which oppose one anothen An electromagnetic wave is applied to the structure which is capable of establishing multipactor action between the sec-` ondary electron emissive surfaces when the power level of the electromagnetic wave is greater than the preselected power level but which is incapable of establishing multipactor action when the power level of the electromagnetic wave is less than the preselected power level. Means, such as an electron gun, is provided to supply at least a predetermined number of electrons having at least a preselected kinetic energy to the waveguiding structure in the vicinity of the secondary electron emissive surfaces to insure immediate commencement of multipactor action in response to power above the preselected level.

Additional objects, advantages, and characteristic features of the present invention will become readily apparent from the following detailed description of a preferred embodiment thereof when considered in conjunction with the accompanying drawings in which:

FIG. 1 is a longitudinal view, partly in section, of a multipactor switch in accordance with the present invention; t

FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. l;

FIG. 3 is an end view of the device of FIG. 1 as seen from line 3 3;

FIG. 4(a) is a timing waveform showing typical input power pulses to a multipactor device;

FIG. 4(b) shows the power output from a prior art multipactor device in response to the timing waveform of FIG. 401); and

FIG. 4(c) illustrates the power output from a multipactor device according to the present invention in response to the waveform of FIG. 4(a).

Referring to the drawings with greater particularity, and especially to FIGS. 1 and 2, a power switching device in accordance with the` present invention may be seen to include a-metalhousing 10 which deiines a waveguiding surface 12 for propagating electromagnetic waves within a predetermined frequency range. Although the wave propagating surface 12 may be of various congurations such as a rectangular waveguide, a ridged waveguide or an interdigital transmission line, in the illustrated embodiment the waveguide 12 takes the form of a comb-like bandpass filter structure. Thus, the wave propagating structure 12 may comprise a rectangular waveguide in which a plurality of teeth 14, in the shape of rectan-gular parallelepipeds, project inwardly from spaced points along one of the waveguide walls. The teeth 14 are sequentially disposed and aligned with one another along a longitudinal axis of the waveguide 12.

The inner end surface of each tooth 14 is coated with a layer 16 of a secondary electron emissive material, i.e., a material having a secondary electron emission coefficient greater than unity. The wall of the waveguide 12 facing the inner end surfaces of the teeth 14 is also provided with a coating 18 of a secondary electron emissive material. Examples of suitable materials which may be used for the secondary electron emissive coatings 16 and 18 are a silver-ma-gnesium alloy containing approximately 98.5% silver, 1.3% magnesium and 0.2% impurities; and a copper-beryllium alloy consisting of approximately 98 copper and 2% beryllium.

As is shown in FIGS. 1 and 2, each tooth 14 has a height h, a width w, and a length l. The spacing between successive teeth 14 is designated by s, while the distance between the coatings 16 and 18 is denoted as d. As may be seen from FIG. 2, the waveguide 12 has a width a, with each tooth-14 residing essentially equidistant `from both side walls of the waveguide 12. In an illustrative example of particular dimensions suitable for operation at X-Band frequencies, 1:.15 inch, w=.05 inch, 11:.2 inch, d=,.01 inch, and I=s=.1 inch.

Each end of the waveguide 12 opens into a transition waveguide section 20, which may be a standard rectangular X-Band waveguide, for example, and which waveguide section has a greater height and width than the waveguide 12. A tuning element 22 is disposed in each of the waveguide transition sections 20 adjacent the respective entrances to the waveguide 12. The tuning elements 22 have a variable penetration depth into the respective waveguide sections 2 0 in order to adjust the impedance match between the waveguides 12 and 20.

The end of each waveguide section 20 remote from the waveguide 12 is secured to a circular waveguide section 24 by brazing or welding. A circular disk 26 of a dielectric material such as forsterite or alumina is sealed to the inner walls of the waveguide 24 by metallizing the lateral edge of the disk 26 and brazing it to the waveguide. Thus, the disk 26 is able to function as a vacuum window which enables the interior waveguides 12 and 20 to be maintained at a reduced pressure, -for example -6 microns Hg, while allowing microwave energy to pass freely into and out of the waveguides 12 and 20 through the disks 26.

The end of each waveguide section 24 is provided with a rectangular coupling ange 28 for use in coupling to an external waveguide or other microwave transmission line (not shown). Holes 30 .(see FIG. 3) may be provided in the corners of the coupling flange 28 to receive bolts or screws for attaching the iiange 28 to the external waveguiding circuitry. A rounded rectangular central aperture 32 in each coupling flange 28 serves -as an input or output port for the Switch, the port 32 at one end of the device functioning as the input port and the port 32 at the other end serving as the output port.

In accordance with the principles of the present invention, in order to irradiate the multipactor region of the switch with electrons and thereby insure an essentially instantaneous commencement -of multipactor action in response to an input signal above a given power level, an electron gun -40 is provided. As may best be seen in FIG. 2, the electron gun 40 includes a housing of a metal such as molybdenum having an enlarged tubular portion 42 disposed externally of the switch housing 10 and a coaxially aligned smaller tubular portion 44 primarily disposed in a cylindrical bore hole 46 in the switch housing 10. The cylindrical bore 46 extends through the housing 10 in a Adirection perpendicular to the longitudinal axis of the waveguide 12 to a side wall of the waveguide 12 where a junction with the waveguide 12 is provided. The diameter 0f the bore 46 is substantially greater than the distance d between secondary electron emissive surfaces 16 and 18 and, as shown, a portion of the circumferential surface of the bore 46 extends slightly beyond the wall of the waveguide 12 containing the coating 18 in a direction parallel to the tooth height h.

An electron emissive cathode 48, which may be a thoriated tungsten coil, is disposed within the tubular housing 44 near its end adjacent the waveguide 2. The cathode 48 is supported by and is electrically connected to a pair `of leads 56 and 52, which in turn are mounted on and extend through a circular ceramic disk 54. The cathode supporting disk 54, which also `functions as a vacuum window, is sealed to the inner surface of a metal ring 56 of molybdenum for example. The enlarged portion 42 of the gun housing deines a iirst section 58 of an inner diameter essentially equal kto the outer diameter of the ring 56 and a second section 60 of an inner diameter less than that of the section 58, the section 60 being axially located between the section 5S and the smaller portion 44 of the gun housing. The ring 56 is disposed within the outer section 58 so thatthe axially inner end of the ring 56 abuts against the axially outer end of the radially inwardly projecting flange defined by the housing section 60, the ring 56 being secured to the housing section 58 by `means of a heliarc weld.

A wire screen 62, of gold-plated tungsten for example, is mounted across the tubular gun housing 44 at its end adjacent the waveguide 12 in order to improve the impedance match between the waveguide 12 and the electron gun 40 as seen by electromagnetic waves propagating along the waveguide 12. The screen 62 lalso shields the waveguide 12 from noise generated by the gun 40. A heater voltage source 64,y providing 6.3 v. AC for example, is connected between the cathode leads 50 and 52 to provide heating current suiiicient to cause thermionic emission of electrons from the cathode 48. A DC voltage source `66 is connected between the cathode 48 and the housing 10 in order to bias the cathode 48 negatively with respect to the waveguide 12 and thereby direct the emitted electrons into the waveguide 12. In order to impart sufiicient kinetic energy to the electrons, the magnitude of the Voltage provided by the source 66 should be at least volts, and preferably around 150 volts. The electron current in the stream emitted by the cathode 48 should be in a range essentially between l0 microamperes and 5 milliamperes.

In order to gain a better understanding of the present invent-ion, the following mathematical analysis is included setting forth parameter relationships necessary to commence and sustain multipactor action, and which relationships may be employed in designing a multipactor switch in accordance with the invention. The analysis assumes that an electric field intensity E is established between the secondary electron emissive surfaces 16 and 18 in a direction perpendicular to the surfaces 16 and 18 i.e., along the direction of the surface spacing d in FIG. 1. It is further assumed that electron motion occurs entirely along the aforementioned direction of the E field, and which direction shall be referred to as the x-direction.

From classical dynamics the force F on an electron subjected to the foregoing environment is given by:

F: QE 1) where q is the electronic charge and E is the electric iield intensity. Using Newtons Second Law, Equation 1 can be rewritten as:

where mi is the mass of an electron. Assuming a sinusoidal variation in E, Equation 2 becomes:

dtzm S111 wt (3) where E is the maXimum amplitude and w is the frequency of the electric eld E. Integration of Equation 3 yields:

=qE0 cos OH-K1 dt man Assuming zero initial electron velocity,

K1=qE0/mw (5) Therefore, Equation 4 becomes:

212:@ di mw Integrating Equation 6 gives:

(l-cos wt) (6) If x=0 at t=(), K2=O and Equation 7 becomes:

1@ finan) x.- (t w (8) Since the voltagebetween the surfaces 16 and 18'is proportional to the electric field times the surface separation, i.e., V=Ed, Equation 8 may be rewritten as:

where the electric iield has a frequency f=w/21r and where n is any integer. It should be pointed out, however, that multipactor action will still occur even though the electron transit times are not exactly equal to (2n-1) half cycles and even if the electrons are slightly out of phase at the beginning of the cycle.

Substituting Equation 10 into Equation 6, the electron velocity when x=d becomes:

f@ QVo dt x=d*rrmfd (1l) Similarly, substituting Equation 10 into Equation 9 and letting x=d yields:

t (2n-110W) transversing the gap d may be determined from:

where v is the velocity of the electron upon reaching the opposite surface. Since the velocity v may be determined from Equation 11, substitution of Equation 11 into Equation 13 yields:

Uhm( @V0 uw qu 2 wmfd 1r 41rmf2d2 (14) Solving lEquation 12 for V0, and substituting the result for the V0 inside the parenthesis on the right side of the Equation 14 gives:

2QV0 1r(2n*1) (15) Converting to electron volts:

l Urn-i) eV' (16) Where U1 is the stored energy per unit length and vg is the group velocity of the electromagnetic wave which may be determined from an wdiagram for the particular slow-wave structure in question. The energy stored per unit length in a multipactor switch according to FIGS. 1

and 2 is given by:

U1: U/z=1/2c1r/02 (18) where C1 is the capacitance per unit length between the surfaces 16 and 18. C1 may be determined from:

CFM/d (19) where e is the dielectric constant of the material (air in this instance) between the secondary electron emissive surfaces, and d and w are as shown in FIGS. 1 and 2. Substituting Equation 19 into Equation 18, substituting the resultant equation into Equation 17 and solving for V0 yields:

V11=(2P1d/ewvg)/n (2.0) Substituting Equation 2O into Equation 12 gives:

a2= i 2n1 2131) m 41rf2 ewvg (21) and substitution of Equation 20 into Equation 16 results in:

Equations 21 and 22 are useful in the design of a low level multipactor switch having a coniiguration illustrated in FIGS. 1 and 2. The minimum power level at which multipactor action is to occur is one of the design parameters which is given. Since vg may be determined from an wdiagram, and w and f selected according to the desired frequency characteristics of the slow-wave structure, the surface spacing d and the minimum energy U necessary to sustain multipactor action may be calculated. For optimum results it has been found that n should equal either one or two and U should vary between essentially 200 and 2000 electron volts depending on the particular secondary electron emissive material employed.

Equation 22 deiines the minimum electron energy U necessary for initiating multipactor action in terms of a specified power level. Therefore, when the input power is below this critical, or threshold, level, multipactor action does not commence, and power is transmitted through the switch with essentially no attenuation except for cold insertion loss. On the other hand, when the input power is greater than the threshold level, electrons are accelerated across the gap d and emit secondary electrons which are in phase with the electric field so that a current is rapidly-established between the surfaces 16 and 18.

The current density is limited by space charge forces which shift some of the electrons out of synchronism with the electric field and thereby establish an equilibrium condition. As the electrons are accelerated back and forth across the gap d, they absorb energy from the electric field, and this energy is dissipated when the electrons strike the surfaces 16 and 18. Thus, when the input power to the switch exceeds the threshold level, multipactor action occurs which removes power from the electromagnetic waves, thereby limiting the output power to essentially the threshold level.

When a multipactor switch is constructed in the manner discussed above, except with the omission of the electron gun 40 and its associated circuitry, the aforedescribed power limiting operation does not commence immediately after turn-on of the device. Specifically, a time delay exists between the time when power greater than the threshold level is first applied to the device and the time when multipactor action commences which limits the output power to thetlireshold level. This time delay, termed a turn-on transient, is illustrated in FIGS. 4(a) and (b). In FIG, 4(a) an exemplary sequence of input power pulses are shown in which four pulses 70 (each of a power level less than the threshold power Pm necessary to commence multipactor action) are first received followed by a series of pulses 72 (each of a power level greater than the threshold power Pth).

A typical output response to the pulses of FIG. 4(11) for a multipactor switch of the prior art, i.e., without the electron gun 40 and its associated circuitry, is illustrated in FIG. 4(b). It may be seen from FIG. 4(1)) that the pulses 70 below the threshold power Pm are passed to the output substantially unchanged, but that the first three of the pulses 72 which exceed the threshold level are also passed substantially unchanged. Although multipactor action eventually commences, and the remaining ones of the input pulses 72 are attenuated to essentially the threshold level, as shown by the pulses 72 of FIG. 4(b),

a turn-on transient time delay TD exists during which excessive power passes through the switch. The duration of the time delay TD is directly proportional to the time that the switch had been non-operating and is inversely proportional to the temperature and to the level of input power applied to the switch.

As has been mentioned above, the electron gun arrangement of the present invention removes the aforementioned turn-on transient and thereby insures an essentially instantaneous commencement of multipactor action in response to input power pulses above the threshold level. The electron gun irradiates a portion of the region in which the multipactor action occurs with an electron current of at least a predetermined magnitude,

introduced to the multipactor region, the switch commences multipactor action immediately after the input power level exceeds the threshold power Pth. The effect is illustrated in FIG. 4(c) which depicts the power output from a multipactor switch according to the present invention Vin response to the input pulses of FIG. 4.(a). It may be observed that the first four pulses 70 below the power threshold level Pth are again passed substantially unaffected. However, none of the input pulses 72 which exceed the threshold level Pth are able to pass through the switch, with all of the pulses 72 being reduced to essentially the threshold value Pm as shown by the output pulses 72 of FIG. 4(c). Thus, the multipactor switch of the present invention is able to eliminate the turn-on transient and preclude the passage of all pulses exceeding the threshold power level.

Although the present invention has been shown and described with reference to a particular embodiment, and nevertheless, various changes and modifications obvious to one skilled in the art to which the invention pertains are deemed to lie within the spirit, scope and contemplation of the invention as set forth in the appended claims.

What is claimed is:

1. A multipactor switch comprising: a housing defining a waveguiding surface for propagating electromagnetic wave energy within a predetermined frequency range along a predetermined direction, first and second opposing portions of said waveguiding surface being provided with a coating of a material having a secondary electron emission coefficient greater than unity, the respective coatings on said first and second portions being disposed parallel to said predetermined direction and spaced from one another by a preselected distance, means for establishing in the region between said coatings an alternating electric field having at least a component extending in a direction perpendicular to said coatings, the frequency f of said alternating electric field being related to said preselected distance so as to allow electrons to traverse said preselected distance in a time essentially equal to (2n-1)2f where n is a positive integer, said waveguiding surface defining an aperture extending parallel to said predetermined direction, an electron gun including a thermionic electron emissive cathode disposed externally of said waveguiding surface in the vicinity of said aperture, means for maintaining said cathode at a predetermined negative potential with respect to said waveguiding surface, and means disposed in said aperture for improving the impedance match between said waveguiding surface and said electron gun as seen by electromagnetic waves propagating along said waveguiding surface.

2. A multipactor switch comprising: a housing defining a waveguiding surface for propagating electromagnetic wave energy within a predetermined frequency range along a predetermined direction, first and second opposing portions of said waveguiding surface being provided with ka coating of a material having a secondary electron emission coefficient greater than unity, the respective coatings on said first and second portions being disposed parallel to said predetermined direction and spaced from one another by a preselected distance, means fory establishing in the region -between said coatings an alternating electric field having at least a component extending in a direction perpendicular to said coatings, the frequency j of said alternating electric field being related to said preselected distance so as to allow electrons to traverse said preselected distance in a time essentially equal to (2n-1)2f where n is a positive integer, said waveguiding surface defining an aperture extending parallel to said predetermined direction, an electron gun including a thermionic electron emissive cathode disposed externally of said waveguiding surface in the vicinity of said aperture, means for maintaining said cathode at a predetermined negative potential with respect to said waveguiding surface, and means disposed in said aperture for shielding said waveguiding surface from noise generated by said electron gun.

3. A` switching device for translating electromagnetic waves of less than a preselected power level with substantially no attenuation and for reducing the power level of electromagnetic waves of more than said preselected power level to substantially said preselected power level comprising: a metal element defining a waveguiding passageway capable of propagating electromagnetic waves within a predetermined frequency range between an input port and an output port, said waveguiding passageway having first and second pairs of oppositely disposed parallel walls, one of the walls of said first pair defining a plurality of inwardly projecting portions each having an end surface disposed parallel to the other wall of said first pair, said other wall of said first pair and each of said end surfaces being provided with a coating of a material having a secondary electron emission coefficient greater than unity, means for applying to said input port an electromagnetic wave of a frequency f and having an electric field perpendicular to said rst pair of walls, said frequency f being related to the distance between the coating on said other wall and the `coating on said end surfaces so as to allow electrons to traverse said distance in a time essentially equal to (2n-1)/2f where n is a positive integer, said element defining an auxiliary pas` sageway disposed along a direction perpendicular to said second pair of walls and extending through one of said second pair of walls to provide a junction with said wavelguiding passageway, said auxiliary passageway extending beyond said one Wall of said first pair and said end surfaces along a direction perpendicular to said first pair of walls, means for maintaining said waveguiding passageway and said auxiliary passageway at a pressure less than atmospheric pressure, a therrnionic electron emissive cathode disposed in said auxiliary passageway in the 15 vicinity of said junction, means for maintaining said cathode at a predetermined negative potential with respect to said metal element, and a wire screen mounted across said auxiliary waveguiding passageway in the vicinity of said junction.

References Cited UNITED STATES PATENTS 2,674,694 4/1954 Baker 333--13 3,278,865 10/1966 Forrer S33-98 3,312,857 4/1967 Farnsworth 313-104 FOREIGN PATENTS 851,881 10/1960 Great Britain.

HERMAN KARL SAALBACH, Primary Examiner.

S. CHATMON, J R., Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No. 3,354,349 November 21, 1967 Kenneth L. Horn It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 16, for "2" read 12 column 5, lines 4l to 43, the equation should appear as shown below instead of as in the patent:

Column 7, line 74, strike out "and"; column 8, lines 19 and 47, for "(2nl)2f", each occurrence, read (Zrbl) T Signed and Sealed this 18th day of February 1969.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A MULTIPACTOR SWITCH COMPRISING: A HOUSING DEFINING A WAVEGUIDING SURFACE FOR PROPAGATING ELECTROMAGNETIC WAVE ENERGY WITHIN A PREDETERMINED FREQUENCY RANGE ALONG A PREDETERMINED DIRECTION, FIRST AND SECOND OPPOSING PORTIONS OF SAID WAVEGUIDING SURFACE BEING PROVIDED WITH A COATING OF A MATERIAL HAVINGA SECONDARY ELECTRON EMISSION COEFFICIENT GREATER THAN UNITY, THE RESPECTIVE COATINGS ON SAID FIRST AND SECOND PORTIONS BEING DISPOSED PARALLEL TO SAID PREDETERMINED DIRECTION AND SPACED FROM ONE ANOTHER BY A PRESELECTED DISTANCE, MEANS FOR ESTABLISHING IN THE REGION BETWEEN SAID COATINGS AN ALTERNATING ELECTRIC FIELD HAVING AT LEAST A COMPONENT EXTENDING IN A DIRECTION PERPENDICULAR TO SAID COATINGS, THE FREQUENCY OF SAID ALTERNATING ELECTRIC FIELD BEING RELATED TO SIAD PRESELECTED DISTANCE SO AS TO ALLOW ELECTRONS TO TRANVERSE SAID PRESELECTED DISTANCE IN A TIME ESSENTIALLY EQUAL TO (2N-1) 2F WHERE N IS A POSITIVE INTEGER, SAID WAVEGUIDING SURFACE DEFINING AN APERTURE EXTENDING PARALLEL TO SAID PREDETERMINED DIRECTION, AN ELECTRON GUN INCLUDING A THERMIONIC ELECTRON EMISSIVE CATHODE DISPOSED EXTERNALLY OF SAID WAVEGUIDING SURFACE IN THE VICINITY OF SAID APERTURE, MEANS FOR MAINTAINING SAID CAHTODE AT A PREDETERMINED NEGATIVE POTENTIAL WITH RESPECT TO SAID WAVEGUIDING SURFACE, AND MEANS DISPOSED IN SAID APERTURE FOR IMPROVING THE IMPEDANCE MATCH BETWEEN SAID WAVEGUIDING SURFACE AND SAID ELECTRON GUN AS SEEN BY ELECTROMAGNETIC WAVES PROPAGATING ALONG SAID WAVEGUIDING SURFACE. 